Many electronic devices include one or more substrates having a circuit assembly assembled thereon. In order to manufacture high quality electronic devices in a cost effective manner, it is necessary to assemble the circuit assembly to the substrate at a high speed and with great precision. These goals are achieved through an automated circuit assembly process that is quality controlled.
One such automated circuit assembly process employs an automated printing machine to print solder paste on predetermined sites of a substrate and an automated placement machine to place an electronic part such that its leads contact the solder paste. To ensure that the electronic part is accurately placed, the automated circuit assembly process employs a vision inspection system to inspect the electronic part. The inspection system detects the location of the electronic part and compares the detected location to a prestored ideal location. From the difference in the detected and ideal locations, the inspection system determines a longitudinal axis or x-coordinate offset, a transverse or y-coordinate offset, and an offset value indicating the amount of rotation of the electronic part. When any of the offsets, individually, exceed a predetermined offset limit, the inspection system rejects the electronic part and indicates that the placement machine must be adjusted.
However, whenever offsets are evaluated individually as described above, there exists a possibility that reliable mechanical and electrical connectivity of the contacts of the electronic part to the site could still be achieved even though one or more of the offsets indicate that the electronic part is misplaced. This is particularly true in an assembly process that employs a single set of ideal locations for the electronic part but uses different suppliers to supply electronic parts with the same electrical function but slightly different mechanical features.
Therefore, what is needed is an improved inspection method that bases part rejection on mechanical and electrical connectivity capability rather than on the relative location of the part, thereby minimizing excessive rejection rates that can seriously impact capacity and throughput for the automated circuit assembly process.